The 2013 El Reno tornado was an extremely large, intense, and erratic tornado that occurred and hit a rural part of Central Oklahoma during the early evening of May 31, 2013. This rain-wrapped, multiple-vortex tornado was the widest tornado ever recorded, peaking at 2.6 miles across. It was part of a larger weather system that produced dozens of tornadoes over the preceding days. The tornado initially formed at 6:03 p.m. CDT (23:03 UTC) about 8.3 miles (13.4 km) west-southwest of El Reno, rapidly growing in size and becoming more violent as it tracked through central Canadian County. Remaining over mostly open terrain, the tornado did not impact many structures; however, measurements from mobile weather radars revealed extreme instantaneous winds in excess of 313 mph (504 km/h) within sub-vortices. These are among the highest recorded wind speeds, just slightly lower than the wind speeds of the 1999 Bridge Creek–Moore tornado. As it crossed U.S. 81, it had grown to a record-breaking width of 2.6 miles (4.2 km), beating the previous width record set in 2004. Turning northeastward, the tornado soon weakened. Upon crossing Interstate 40, the tornado dissipated around 6:43 p.m. CDT (23:43 UTC), after tracking for 16.2 miles (26.1 km). It avoided affecting more densely populated areas near and within the Oklahoma City metropolitan area.
The tornado killed four storm chasers (three professional and one amateur), the first known deaths in the history of storm chasing. Although the tornado remained over mostly open terrain, dozens of storm chasers unaware of its immense size and erratic movement were caught off-guard. Near U.S. 81, TWISTEX scientist and engineer Tim Samaras, along with his son Paul and research partner Carl Young, died in the tornado. Paul Samaras and Young were ejected from their Chevrolet Cobalt by the storm's sub-vortex, while Tim was still buckled in the passenger's seat. Local resident Richard Henderson, who decided to follow the storm, lost his life in that same area. He snapped a picture of the tornado from his cellular phone before it struck him. Other chasers, including Mike Bettes of The Weather Channel and Reed Timmer, either were injured or had their vehicles damaged. A Doppler on Wheels-based analysis of how the tornado impacted these teams revealed that they were hit by an intense internal sub-vortex. Overall, the tornado was responsible for eight fatalities and 151 injuries. Due to the ferocity and sheer size, as well as its irregular movement and the deaths linked with this tornado, it has become one of the most studied and infamous tornadoes ever. The National Weather Service referred to the tornado as "the most dangerous tornado in storm observing history".
Alongside rush hour traffic, thousands of residents in Oklahoma City attempted to outrun the storm by taking to the roads in an attempt to drive out of the tornado's projected path. By attempting to escape the storm by vehicle, in direct contrast to the recommended plan of action, residents put themselves at great risk from the storm; had the tornado maintained itself and passed over the congested freeways, more than 500 lives could have been lost.
On May 31, 2013, a prominent mid-to-upper level trough and closed mid-level low pressure area moved east-northeastward, with a lead upper low pivoting over the Dakotas and Upper Midwest region. A moderately strong polar jet moved east-northeastward over the southern Rocky Mountains to the southern Great Plains. With a broad influence of moderately strong cyclonic flow aloft, the air mass was expected to become unstable across much of the southern Great Plains, through the Upper Midwest and Mississippi Valley, by the afternoon.
Dewpoint values ranged from the upper 60s °F (20 °C) to the lower 70s °F (20–22 °C), with temperatures in the low to mid-80s °F (27–30 °C), and CAPE (a meteorological concept used to predict the force and power of potential storms and tornadoes) predicting referring to values ranging from 3500 to 5000 J/kg. Deep layer wind shear speeds of 45–55 kn (52–63 mph) would enhance storm organization and intensity. These factors, along with CAPE values in excess of 4000 J/kg and an embedded speed maxima rotating around the southern periphery of the low, made the threat of significant severe thunderstorms increasingly likely. These ingredients were present ahead of a cold front extending from the low from the eastern Dakotas southwestward to western Oklahoma, and ahead of a dry line extending from western Oklahoma southward into western north and west-central Texas. The most intense severe weather activity was expected across the southern Great Plains, specifically central and eastern Oklahoma, during the afternoon. As such, the Storm Prediction Center (SPC) issued a moderate risk of severe thunderstorms during the early morning of May 31 from southeastern Missouri to southwestern Oklahoma. The degree of wind shear, moisture and instability within the warm sector favored the development of discrete supercells. Very large hail and tornadoes were expected with the supercells, with the possibility of a few strong to violent tornadoes.
At 3:30 p.m. Central Daylight Time (CDT) early that afternoon, the SPC issued a Particularly Dangerous Situation Tornado Watch from southwestern through northeastern Oklahoma, surrounding the Interstate 44 corridor.
A quasi-linear complex of thunderstorms began developing near the Highway 81 corridor west of Oklahoma City between 4:00 and 4:45 p.m. CDT, and rapidly reached severe intensity. At 5:33 p.m. CDT (22:33 UTC), the National Weather Service Weather Forecast Office in Norman, Oklahoma issued a tornado warning for Canadian County, prompted by increasingly strong tornadic circulation exhibited in the southernmost supercell of this complex.
At 6:03 p.m. CDT (23:03 UTC), a large wall cloud formed and produced a tornado, initially in the form of several smaller sub-vortices, 8.3 miles (13.4 km) west-southwest of El Reno. The tornado ultimately attained EF3 intensity during its existence, according to ground surveys. At 6:15 p.m. CDT (23:15 UTC), the tornado deviated from its easterly path, doing a full loop before resuming its ESE motion. During its motion in this direction, two people were struck by the tornado and killed (an amateur storm chaser and a local resident). At 6:23 p.m. CDT (23:23 UTC), as the tornado passed south of El Reno across U.S. 81, it grew to an unprecedented width of 2.6 miles (4.2 km), becoming the widest known tornado ever recorded, simultaneously killing three professional storm chasers and another motorist driving nearby. At 6:28 p.m. CDT (23:28 UTC), the storm began moving into more densely populated areas of Canadian County while maintaining its intensity. This prompted the National Weather Service office in Norman to issue a tornado emergency for Yukon, Richland, Wiley Post Airport, Bethany, The Village, and eastern El Reno, as the tornado was projected to track toward western portions of the Oklahoma City metropolitan area. Within minutes, the tornado turned northeast and soon passed directly over Interstate 40 at around 6:42 p.m. CDT (23:42 UTC), killing two people in a minivan that was thrown as a result of the tornado. Shortly thereafter, the tornado lifted off the ground as it neared Banner Road. Overall, the tornado was on the ground for 40 minutes along a 16.2-mile (26.1 km) path.
A strong anticyclonic multi-vortex satellite tornado also formed southeast of the primary tornado at approximately 6:28 pm CDT (23:28 UTC), and remained on the ground for 15 minutes before lifting at 6:43 p.m. (23:43 UTC). Such companion tornadoes tend to be observed with especially large and intense tornadoes, although this was the first documented multiple-vortex anticyclonic tornado of this kind.
The intensity of the tornado has been a subject of internal debate within the National Oceanic and Atmospheric Administration. The agency uses the Enhanced Fujita Scale to rate and assess tornado intensity based on the damage left behind. This excludes the use of supplementary measurements, such as those from mobile radar, in concluding a tornado's intensity. Initially receiving an official EF3 rating based on damage, the El Reno tornado was subsequently upgraded to a radar-estimated EF5 rating, the highest on the scale, based on data from a mobile radar. The University of Oklahoma's RaXPol mobile Doppler weather radar, positioned at a nearby overpass, measured winds preliminarily analyzed as in excess of 296 mph (476 km/h). These winds are considered the second-highest ever measured worldwide, just shy of the 321 mph (517 km/h) recorded during the 1999 Bridge Creek–Moore tornado.